1. Field of the Invention
The present invention relates to a multiple antenna communication system. More particularly, the present invention relates to a method and apparatus for feeding back channel information.
2. Description of the Related Art
Research is being conducted on a Multiple-Input Multiple-Output (MIMO) scheme. In comparison with a system using a single antenna, a system using multiple antennas in both transmitting and receiving ends may increase channel transmission capacity in proportion to a number of antennas without additionally allocated frequencies or Transmit (Tx) power.
The MIMO scheme may include a Single-User MIMO (SU-MIMO) scheme and a Multi-User MIMO (MU-MIMO) scheme. The SU-MIMO scheme is a point-to-point MIMO scheme in which communication is performed by concurrently allocating the same radio resources (e.g., time, frequency, etc.) to a single user. The MU-MIMO scheme is a point-to-multipoint MIMO scheme in which communication is performed by concurrently allocating the same radio resources to several users. Space-Time Transmit Diversity (STTD) and Spatial Multiplexing (SM) are examples of the SU-MIMO scheme on which many studies have been conducted up to now. Per-User Unitary Rate Control (PU2RC), Space Division Multiple Access (SDMA), and Dirty-Paper Coding (DPC) are examples of the MU-MIMO scheme which are researched.
The MIMO scheme may also include an Open-Loop MIMO (OL-MIMO) scheme and a Closed-Loop MIMO (CL-MIMO) scheme according to whether a Base Station (BS) uses feedback to know channel information. Both of the OL-MIMO and CL-MIMO schemes may be used in the SU-MIMO scheme. However, only the CL-MIMO scheme may be used in the MU-MIMO scheme since the BS may be aware of the channel information. In the CL-MIMO scheme, the channel information may be reported by a Mobile Station (MS) to the BS, and an amount of feedback information generally increases in proportion to the number of antennas.
As described above, the BS is generally aware of the channel information through feedback in the MU-MIMO scheme. In this case, two methods are used. In a first method, a channel coefficient is fed back after being quantized (e.g., in a case of Frequency Division Duplex (FDD)) or the BS estimates a channel by using a sounding signal in uplink (e.g., in a case of Time Division Duplex (TDD)). In the first method, the BS may use information on the channel, and thus there is an advantage in that a more correct and sophisticated MU-MIMO scheme may be used. However, the first method has a disadvantage in that a feedback overhead is large. In a second method, a predefined codebook is used. The MS selects a preceding matrix from the predefined codebook and feeds back only an index of the preceding matrix, thereby decreasing a feedback amount. In this case, an index of the codebook is Channel Direction Information (CDI) that indicates only a directivity of a channel. Therefore, there is a need to additionally feed back Channel Quality Information (CQI) such as a Signal to Interference and Noise Ratio (SINR).
In the MU-MIMO scheme, the PU2RC scheme is a conventional unitary preceding scheme. Information to be fed back in the PU2RC scheme is a preceding matrix, information on a data stream, and information on an SINR at that time. A method of feeding back the information may be selected variously. An optimization method is a method in which an SINR is fed back for all preceding matrixes included in a codebook and for all layers (i.e., a data stream transmitted from a Tx antenna) and thereafter a BS compares several possible combinations to determine information to be fed back. However, this method is difficult to implement due to a large amount of feedback. As a way of reducing the amount of feedback, there is a method of feeding back a Post Detection (PD)-Carrier to Interference and Noise Ratio (CINR) by selecting an index of a preceding matrix and a layer index included in the preceding matrix. However, the method of selecting and feeding back the index of the preceding matrix and the index of the layer index included in the preceding matrix has a limit in increasing a Quality of Service (QoS) of the MS, and system capacity may deteriorate due to limited channel information when a small number of MSs are used.
Accordingly, in the conventional case, transmission requirements are difficult to be satisfied when the same feedback method is used in various MIMO transmission environments.